Neuron types in the rat lateral superior olive and developmental changes in the complexity of their dendritic arbors.

The lateral superior olive (LSO), a conspicuous mammalian brainstem nucleus that is involved in sound localization, has become a model system for investigating the formation of topographically organized inhibitory and excitatory connections. In experiments employing intracellular injections of Lucifer yellow or neurobiotin into lightly fixed brain slices, we have examined the soma-dendritic morphology of 483 LSO neurons of rats between postnatal day (P) 4 and P36. A detailed analysis of the shape and complexity of dendritic arbors was performed in 238 neurons in order to identify different cell classes and to determine whether age-related changes occur that may relate to a topographical refinement. Regardless of age, seven classes of LSO neurons were identified, more than had been delineated previously with the Golgi technique. Bipolar neurons and multipolar neurons comprised the two major cell types, whereas small multipolar cells, banana-like cells, bushy cells, unipolar cells, and marginal cells were found less frequently. Age-related changes were analyzed in bipolar and multipolar neurons, and several modifications of their dendritic arbors were observed that are in accordance with a refinement of topography. For example, at P4, bipolar and multipolar cells had relatively broad dendritic arbors, with an average of 140 and 138 dendritic end branches, respectively. During further development, their numbers became drastically reduced by about 80%, such that an average of less than 30 endpoints remained by P36. As the dendritic arbors became smaller specifically along the transverse axis of the LSO, they became confined to a smaller frequency area. We conclude from our results that considerable remodeling takes place in the LSO and that the selective loss of dendritic branches may be a morphological correlate for the formation of exquisite tonotopy.

Influence of the neuropeptide somatostatin on the development of dendritic morphology: a cysteamine-depletion study in the rat auditory brainstem.

We investigated the functional role of somatostatin during early ontogeny of the brain, when the neuropeptide as well as its receptors are heavily expressed in the auditory brainstem. Rat pups received a daily injection of cysteamine which, when applied at low concentrations, most selectively depletes somatostatin. Neurons from the lateral superior olive, an auditory brainstem nucleus which transiently receives a dense somatostatinergic input, were intracellularly labeled at postnatal day 14 or 18. The dendritic morphology of these neurons was then analyzed quantitatively and compared with neurons from controls. Cysteamine treatment induced a reduction of the number of dendritic end points by more than 50%. At postnatal day 14, for example, controls and somatostatin-depleted animals had an average of 58 and 28 end points, respectively. The number of primary dendrites was also significantly reduced by cysteamine. In contrast, the size of the somata, the orientation of the dendritic trees within the lateral superior olive, the dendritic areas, and the cross-sectional size of the lateral superior olive were not altered. These results indicate that somatostatin depletion during early development has profound effects on the maturation of dendritic morphology. The selective influence on the dendritic trees suggests that somatostatin acts as an endogenous trophic peptide and promotes the achievement of dendritic complexity.